Abstract

Self-organized periodic structures have been observed on the surface of the ablation craters of Ge-S based chalcogenide glass produced after irradiation by a focused beam of a femtosecond Ti:sapphire laser (1 kHz, 34 fs, 806 nm). Scanning electron microscopy and atomic force microscopy images of irradiated spots show a periodic structure of ripples with a spatial period of 720 nm (close to the wavelength of fs laser pulses) and an alignment parallel to the electric field of light. With an increasing number of pulses, from 5 to 50 pulses, a characteristic evolution of ripples was observed from a random structure to a series of generally aligned peaks-and-valleys self-organized periodic structures. Additionally, at the center of the ablated spot, micro-domains appear where the ripples are still regular but are assembled in a more complex fashion. The experimental observations are interpreted in terms of strong temperature gradients combined with interference of the incident laser irradiation and a scattered surface electromagnetic wave.

SEM images of an ablated region exposed to N = 50 successive pulses at a peak pulse fluence of 0.42 J/cm2. (a) Profile of ablated region. (b) Full image of the exposed region. The diameter of the affected area is ≈620 µm. The ring represents the region in which straight self-organized structures are formed. (c) Zoom on the self-organized structures observed inside the ring. The arrows indicate the polarization of the electric field.